The long-term goal of this research is a better understanding of the molecular mechanism and regulatory processes involved in the maintenance of the constant pool of plasma iron that is essential for the biosynthesis of hemoglobin and other important iron- containing systems. It is hoped that studies of this nature will provide insight into the molecular bases of metabolic disorders such as iron deficiency anemia, one of the most common deficiency disorders in humans, and hemochromatosis, the excess storage of iron. The specific objective of this study is to determine, on a molecular level, how the mucosal cell rapidly adjusts to decreases in the iron content of the diet and to decreases in the iron stores in the body to permit the absorption of a greater fraction of the iron present in the diet. Both important phases of iron absorption, uptake into the mucosal cell and processing within the cell, have been implicated as playing a role in this adjustment. Upon transferring animals from a diet of normal iron content to one of low iron content, detailed analyses will be made of the molecular changes that occur both in the uptake of iron into the mucosal cell and in the processing of iron within the mucosal cell. A comparison of these changes as well as a comparison of how well these changes correlate with alterations in the actual absorption of a test dose of iron and in the iron status of these animals should provide insight into the relative regulatory importance of the two important phases of iron absorption.